In a conventional optical transmission system, a regenerating optical repeater has been used to regenerate a signal attenuated through optical fibers. The regenerating optical repeater converts an optical signal to an electric signal, and then discriminates the digital signal in order to regenerate an optical signal. However, this regenerating optical repeater had a disadvantage in that operation speed of an electronic component was limited when converting an optical signal to an electric signal, and power consumption was increasing as transmission speed became higher for example.
For solving this problem, a fiber laser amplifier or a semiconductor laser amplifier was widely used. The fiber laser amplifier boosts signal light by passing through optical fibers doped with rare earth element (e.g., erbium or praseodymium) excited by a pump light. The fiber laser amplifier and the semiconductor laser amplifier can directly amplify signal light and thus is not limited by the electric processing speed which has been a problem in a regenerating optical repeater. In addition, the fiber laser amplifier and the semiconductor laser amplifier also have an advantage that the configuration is relatively simple. However, these laser amplifiers do not have a function to reshape a degraded pulse waveform of a signal light. Furthermore, in the case of these laser amplifiers, amplified spontaneous emission, which is generated unavoidably and randomly, is irrespectively mixed with a signal component, thus causing a reduction of the S/N ratio of the signal light by at least 3 dB before and after the amplification. They cause an increase of a bit error rate and the decrease of the transmission quality in the digital transmission systems.
As an Amplifier for solving the limit on the conventional laser amplifiers as described above, a phase sensitive amplifier (PSA) has been studied. This phase sensitive amplifier has a function to reshape a signal light waveform degraded due to the dispersion of the transmission fibers. The phase sensitive amplifier also, in principle, is able to keep the same S/N ratio of the signal light without degradation before and after the amplification because the phase sensitive amplifier can suppress the spontaneous emission with a quadrature phase irrespective of the signal.